Paulo Atsushi Suzuki

Kinetics of thermal decomposition of titanium hydride powder using in situ high-temperature X-ray diffraction (HTXRD)

The thermal decomposition of titanium hydride powder (d-phase) to titanium (a-phase) was investigated by means of thermogravimetric analysis (TGA) and high-temperature X-ray diffraction (HTXRD) in high vacuum. The d-to-a phase transformation was followed in situ by HTXRD at temperatures varying from room temperature up to 1000 °C. The transformation was also analyzed as a function of time at isothermal conditions from 450 to 650 °C. The results of TGA show that the decomposition of the titanium hydride becomes significant at about 450 °C. Above 500 °C the decomposition is completed in times shorter than 50 minutes. The apparent activation energy for hydrogen desorption was found to be 63 ± 6 kJ.mol-1.

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nbss (solid solution), Nb3B2, NbB, Nb5B6, Nb3B4, NbB2, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb3B4, and NbB2 phases. However, the stability of Nb3B2, Nb5B6, and Nb2B3 phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nbss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb3B2-phase is formed through the peritectoid reaction Nbss+ NbB ↔ Nb3B2. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.

Microstructural characterization of Nb–B–Si alloys with composition in the Nb−Nb5Si2B (T2-phase) vertical section

Multiphase alloys of the Me–B–Si (Me=refractory metal) systems have been considered for structural applications at high temperatures. In this work, the solidification pathway and phase stability of Nb-rich Nb–B–Si alloys were evaluated from arc-melted and heat-treated samples. The primary phases observed in the region of study were Nbss, T2-phase and D88-phase. The as-cast microstructure of all the alloys indicates formation of an Nbss+T2 eutectic in the last part to solidify. From the heat treatment of several alloys, the Nbss−T2 two-phase field was observed to exist at 1700 °C. Based on DTA experiments, the Nbss−T2 two-phase field should be stable up to 2150 °C.

Evaluation of the invariant reactions in the Ta-rich region of the Ta-B system

Invariant reactions in the Ta-rich side of the Ta-B system were evaluated. The alloys were arc melted from pure materials. Pellets prepared from powder mixtures of Ta and TaB were heat treated under vacuum at 1900 and 1950 °C. The microstructures of the alloys were characterized through scanning electron microscopy images and x-ray diffraction (XRD). The microstructural analyses confirmed the type of reactions occurring in Ta-rich region; however, the composition of the liquid phase should be altered: (a) L ↔ TaSS+Ta2B from 23 to 18 at.% B and (b) L+TaB↔Ta2B from 27 to 22.5 at.% B. The formation of Ta3B2 was confirmed as the understoichiometry of Ta2B. The decomposition of Ta2B occurs between 1900 and 1950 °C. The XRD data for alloys with composition between 34 and 50 at.% B indicate the existence of a new high-temperature phase.

Isothermal section of the Ti-Si-B system at 1250 ° C in the Ti-TiSi2-TiB2 region

region) of the ternary Ti-Si-B system at 1250 °C was determined from heat-treated alloys prepared via arc melting. Microstructural characterization has been carried out through scanning electron microscopy (SEM), X-ray diffraction (XRD) and wavelength dispersive spectrometry (WDS). The results have shown the stability of the near stoichiometric Ti

Development and characterization of 3CaO·P2O5-SiO2-MgO glass-ceramics with different crystallization degree

The CaO-P2O5-SiO2-MgO system presents several compounds used as biomaterials such as hydroxyapatite (HA), tricalcium phosphate (TCP) and TCP with magnesium substituting partial calcium (TCMP). The β-TCMP phase with whitlockite structure has interesting biological features and mechanical properties, meeting the requirements of a bioactive material for bone restoration. In this work, the production of Mg-doped TCP, β-TCMP, has been investigated by crystallization from a glass composed of 52.75 wt% 3CaO·P2O5, 30 wt% SiO2 and 17.25 wt% MgO (i.e., 31.7 mol% CaO, 10.6 mol% P2O5, 26.6 mol% MgO and 31.1 mol% SiO2) using heat treatments between 775 °C and 1100 °C for up to 8 h. The devitrification process of the glass has been accompanied by differential scanning calorimetry (DSC), high-resolution X-ray diffraction (HRXRD), relative density and bending strength measurements. The characterization by HRXRD and DSC revealed the occurrence of whitlockite soon after the bulk glass preparation, a transient non-cataloged silicate between 800 °C and 1100 °C, and the formation of diopside in samples treated at 1100 °C as crystalline phases. The overall crystalline fraction varied from 26% to 70% depending on the heat treatments. Furthermore, contraction of the a-axis lattice parameter and expansion of the c-axis lattice parameter of the whitlockite structure have been observed during the heat treatments, which were attributed to the β-TCMP formation with the partial substitution of Ca2+ by Mg2+. Relative densities near 99% and 97% for the glass and glass-ceramics respectively indicated a discrete reduction as a function of the devitrification treatment. Bending strengths of 70 MPa and 120 MPa were determined for the glass and glass-ceramic material crystallized at 975 °C for 4 h, respectively.

Improvement of the Mechanical Properties of Glasses Based on the 3CaO.P2O5-SiO2-MgO System after Heat-Treatment

Glasses based on the 3CaO.P2O5-SiO2-MgO system present high bioactivity aiming the use as bone restorations. On the other hand, the low mechanical properties reduce the importance of this glass aiming the use as restoration bulk specimens. In this work, glass-ceramics were obtained by devitrification of this glass using different temperatures. CaCO3, SiO2, MgO and Ca(H2PO4).H2O were used as starting-powders. Powder mixture was milled/homogenized and melted at 1600°C, for 2h and annealed at 700°C for 4h with cooling rate of 3°C/min. Glass specimens of 151550mm3 were characterized by DTA and XRD analysis. Specimens were heat-treated in different temperatures between 7000C and 1050°C, for 4 hours, obtaining glass-ceramics with different crystallized phase content. Hardness and fracture toughness were determined and correlated with crystalline phase content. The results indicated that crystallization-degree increase with the temperature, and the mechanical properties are improved: Hardness values present increases lower than 20% as function of the crystallization. Fracture toughness may increase 100% as function of temperature (crystallization degree).

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

B composition. The present study concerns the preparation of titanium alloys that contain such phase mixed with α-titanium and other intermetallic phases. High-purity powders were initially processed in a planetary ball-mill under argon atmosphere with Ti-18Si-6B and Ti-7.5Si-22.5B at. (%) initial compositions. Variation of parameters such as rotary speed, time, and ball diameters were adopted. The as-milled powders were pressureless sintered and hot pressed. Both the as-milled and sintered materials were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometry. Sintered samples have presented equilibrium structures formed mainly by the α-Ti+Ti

Efficiency Evaluation of ZrB2 Incorporation in the MgB2 Matrix Phase Using High-Energy Milling

The present study suggests the use of high energy ball milling to mix (to dope) the phase MgB2 with the AlB2 crystalline structure compound, ZrB2, with the same C32 hexagonal structure than MgB2, in different concentrations, enabling the maintenance of the crystalline phase structures practically unaffected and the efficient mixture with the dopant. The high energy ball milling was performed with different ball-to-powder ratios. The analysis of the transformation and formation of phases was accomplished by X-ray diffractometry (XRD), using the Rietveld method, and scanning electron microscopy. As the high energy ball milling reduced the crystallinity of the milled compounds, also reducing the size of the particles, the XRD analysis were influenced, and they could be used as comparative and control method of the milling. Aiming the recovery of crystallinity, homogenization and final phase formation, heat treatments were performed, enabling that crystalline phases, changed during milling, could be obtained again in the final product.

Electrical resistivity of an oxide dispersion strengthened niobium alloy (Nb-0.5 wt%TiO2-ODS)

Abstract The precipitation of TiO 2 during aging of supersaturated Nb-Ti-O alloys was investigated using low-temperature electrical resistivity measurements. Aging temperatures were in the range comprised between 900 and 1100 °C. The four-probe method was used to determine the transport properties up to 5 K and the respective transition temperatures ( T c ) in several Nb-TiO 2 alloys with different oxygen contents. Transmission electron microscopy was also used to characterize these alloys before and after aging. It is believed that the decrease of the interstitial oxygen content in solid solution owing to the TiO 2 precipitation is the main responsible for the drop in the electrical resistivity in the normal state and for the slight changes on T c .